CRUZ BAY, U.S. Virgin Islands  Crews armed with machetes and chainsaws hack through walls of thorny sweet lime plants in this U.S. Caribbean territory, avoiding turpentine trees and guava berries in a bid to remove invasive plants threatening to wipe out the native species.

Fast-growing plants, such as snake root, wild tamarind and gnip trees, grow in such dense thickets that they block indigenous species' access to sunlight and soil, said Dan Clark, an exotic plant specialist with the Virgin Islands National Park in St. John. They affect other plants, insects and birds too.

"These invasive plants have the ability to throw several things out of whack all the way up the chain," Clark said Wednesday as his crew sprayed herbicide on the stumps of invasive plants. "Some of the islands are dense with this stuff."

The Virgin Islands National park boasts some 850 known species of plants, more than 80 percent of which are native, officials said.

With the invasive plants cleared, Clark hopes that the dormant seeds of native species, such as gumbo limbo, fiddlewood and guava berry, will take root in the improved conditions. The work has already paid off: native species have increased by some 300 percent in cleared areas, Clark said.

"Exotic and invasive species are the number two threat to biodiversity, behind habitat loss," said Peter Galvin, a spokesman for the Arizona-based Center for Biological Diversity.

Clark's team plans to spend the month of May removing plants from six uninhabited cays in the U.S. Virgin Islands and across 80 acres (32 hectares) of the 7,100-acre (2,890-hectare) national park in St. John. For three years, they have battled African Guinea grass in Buck Island, near the island of St. Croix.

Islands, such as those in the U.S. Virgin Islands chain, are vulnerable to loss of biodiversity because of the limited amount of forested land.

European settlers in the U.S. territory introduced many nonnative plants as feed for goats in the 17th century. Other invasive plants crept into forests and parks from decorative landscaping outside homes and businesses, Clark said.

Florida has spent millions of dollars trying to eradicate the invasive Brazil pepper plant that was introduced to the area due to its ornamental beauty. In April, Hawaiian officials started using an enormous marine vacuum to remove fast-growing, invasive algae that blocked sunlight to its coral reefs.

Plants aren't the only threat to native vegetation: officials last year captured about 200 feral goats in the Virgin Islands national park whose indiscriminate appetite -- they ate rare flowers and commons weeds -- left behind broad swaths of barren land where they grazed.

Mongooses, often seen scurrying around beach parking lots, have eaten the St. Croix racer snake into extinction. Egg-eating rats threaten seabirds -- especially on small, undeveloped islands where more birds nest, said Renata Platenberg, a biologist with the local fish and wildlife department.

Clark said tackling the problem posed by invasive plants has generated a surprising amount of criticism.

"A lot of people think anything green is good for biodiversity," he said. "The plants I remove actually eliminate biodiversity."

Not all invasive plants are being uprooted. Nonnative Caribbean icons, such as coconut palms and bougainvillea, will be spared since they pose no threat to native species.